This invention relates to reducing the frequency reuse factor in a cellular radio system. In particular, intermodulation interference is reduced by choosing channel numbers with a given channel set.
Cellular radio service is expanding at an explosive rate and will be ubiquitous in the near future. Thus, it is important that the radio spectrum be used to provide service for more customers with little or no extra cost to the service provider. In cellular radio service, a predetermined radio frequency spectrum is allocated to carry the communication between a user""s cellular telephone and the service provider""s base station (the gateway into the cellular switching network.) The spectrum is divided into frequency channels, commonly referred to as xe2x80x9cchannel numbers,xe2x80x9d and are reused by base stations within a service provider""s area. The greater the reuse of frequency channels, the greater the number of cellular radio subscribers that can be simultaneously served. However, one frequency channel cannot be used by two adjacent base stations because each will interfere with one another. While reusing frequency channels more often increases the frequency spectrum efficiency, it also increases the resulting interference. Thus, one skilled in the art balances each factor against the other in order to achieve a compromise.
Radio technology has long recognized the problem of intermodulation (IM) products in radio communications systems (including cellular radio systems). The mixing of two sinusoidal signals having different frequencies in a nonlinear system generates IM products that may interfere with other frequency channels, thus degrading signal quality. IM products correspond to the sum and difference frequency components that are attributed to the xe2x80x9cheterodyning process.xe2x80x9d The heterodyning process is discussed in Carson, Ralph S., Radio Communications Concepts, John Wiley and Sons, 1990, pp. 94-99. Heterodyning does not occur in a completely linear system because no new frequency components can be created. A linear system is a system that has the property of superposition. Superposition means that the output signal of the system resulting from a plurality of input signals can be determined by adding the individual output signals corresponding to each of the plurality of input signals. If the system is not completely linear, new frequency components are created whenever two or more original frequency components exist.
To illustrate the hetrodyning problem, assume that the original frequency components are f.sub.1 and f.sub.2. Third-order nonlinear characteristics generate third-order IM products having frequency components of 2f.sub.1xe2x88x92f.sub.2, 2.sub.2xe2x88x92f.sub.1, f.sub.1+2f.sub.2, 2f sub.1+f.sub.2, 3f.sub.1, and 3f.sub.2. The IM products corresponding to differences are of greater concern because these are more difficult to filter than those corresponding to sums. As an example, let f.sub.1 equal 871.920 MHz and f.sub.2 equal 872.550 MHz. Third-order IM products corresponding to differences are generated at 871.290 MHz (2f.sub.1xe2x88x92f.sub.2) and at 873.180 MHz (2f.sub.2xe2x88x92f sub.1). Third-order IM products corresponding to sums are generated at 2617.020 MHz (f.sub.1+2f.sub.2), 2616.390 MHz (2f.sub.1+f.sub.2), 2615.760 MHz (3f.sub.1), and 2617.650 MHz (3f.sub.2). Higher-order nonlinear characteristics generate higher-order IM products such as the fifth-order and seventh-order IM products. The nth-order IM products have frequency components of pf.sub.1xe2x88x92qf.sub.2 and pf.sub.2xe2x88x92qf.sub.1, where p+q equals n and p is greater than q. Higher-order IM products have a lesser effect than the third-order IM products because the corresponding signal levels have less amplitude. Even-order IM products are generally ignored because the corresponding frequency components can be filtered. (In the above example, the second-order IM product has a frequency component of f.sub.1+f.sub.2, which equals 1744.470 MHz. This frequency is sufficiently removed from the spectrum centered around 850 MHz and thus can be easily filtered.) Third-order IM products are typically responsible for the most adverse effects on other IM products.
The discussion heretofore specified only two frequency components. If more than two frequency components exist, then each possible pair of all frequency components (channel numbers) must be considered, where the collection of channel numbers is commonly called a xe2x80x9cchannel setxe2x80x9d in the art of cellular radio. If the frequency of an IM product is coincident with a channel number of the channel set, then a xe2x80x9chitxe2x80x9d occurs. The total effect is determined by adding the individual effects of each pair. For example, the case in which each of two frequency pairs generate a hit on a given frequency will result in more severe effects than the case in which only one frequency pair generates a hit at the given frequency. Moreover, third-order IM products are also generated by the mixing of three signal components (triplets) having frequencies of f.sub.1, f.sub.2, and f.sub.3, respectively. In such cases, third-order IM products having frequency components of xe2x88x92f.sub.1+f.sub.2+f.sub.3, f.sub.1xe2x88x92f.sub.2+f.sub.3, and f.sub.1+f.sub.2xe2x88x92f.sub.3 are the dominant components. Thus, the total effect of third-order IM products is exacerbated by the presence of these components. The subsequent quantitative assessment of third-order IM products includes only the effects of frequency pairs and not frequency triplets.
In a cellular radio system, full duplex operation is supported so that communication from the serving base station to the mobile subscriber unit (commonly associated with the downlink) and from the mobile subscriber unit to the base station (commonly associated with the uplink) can occur concurrently. The frequency of the downlink (base station to mobile subscriber unit) is spaced 45 MHz from the frequency of the uplink (mobile subscriber unit to base station). For a given call, the serving base station allocates a transmitter and a receiver. Similarly, the mobile subscriber unit tunes its transmitter and receiver to the frequencies associated with the allocated base station equipment. A channel number is associated with both a transmitting frequency and a receiving frequency. For example, the channel number 22 in the B band of the AMPS spectrum is 870.660 MHz for the base station""s transmit frequency (downlink) and is 825.660 MHz for the base station""s receive frequency (uplink). These frequency assignments are the mobile subscriber unit""s receive frequency (downlink) and transmit frequency (uplink), respectively.
IM products are generated if nonlinear characteristics exist at the transmitter, receiver, or structures between the mobile subscriber unit and base station. At the base station, multiple transmitted signals, each having a corresponding frequency value, are combined by an RF combiner or power amplifier so that a common antenna can be utilized. Any nonlinear characteristics of the RF combiner, power amplifier, couplers, filters, duplexers, and cables will also cause signals corresponding to IM products to be transmitted by the antenna. These IM products are detrimental to a call if the frequency of one or more of the IM products is the same as a frequency associated with the call. Even if the RF combiner or power amplifier were completely linear, the receiver of the mobile subscriber unit is exposed to multiple signals having different frequencies. One of the signals corresponds to the frequency associated with the call while the other signals are associated with interference (i.e. calls intended for other mobile subscriber units). If the receiver of the mobile subscriber unit has nonlinear characteristics, IM products are generated. The nonlinear characteristics of the receiver are reflected in the third-order intercept point of the receiver. (Carson, Ralph S., Radio Communications Concepts, John Wiley and Sons, 1990, 94-99.)
Furthermore, the generation of IM products can be associated with factors external to the base station and mobile subscriber unit. In fact, any nonlinear junction or device encountered by a signal in its path of propagation can generate IM products. (Boucher, Neil J., The Cellular Radio Handbook, Quantum Publishing, 1995, p. 100.) Typical nonlinear junctions or devices include bolted tower joints, antenna clamps, tower guy wires, metal fences, chains, and light bulbs. Such factors may be very difficult to identify and to eliminate.
The discussion heretofore addresses the generation of IM products for the downlink (base station to mobile subscriber unit). However, an analogous discussion can be presented for the uplink (mobile subscriber unit to base station). IM products are detected at the base station""s receiver if nonlinear characteristics exist somewhere in the uplink path or at the base station""s receiver and if a plurality of mobile subscriber units are simultaneously transmitting at different frequencies. Signals associated with these different frequencies can mix at points having nonlinear characteristics to generate IM products.
There are several known approaches to diminish the effects of IM products in a radio system. The first approach is to reduce the nonlinear characteristics of the electronic components such as RF power devices. However, this approach may not be technically possible or economically feasible. A second approach is to cancel IM products by inducing a signal that is inverted with respect to the distortions caused by the nonlinear characteristics of the electronic circuitry. Such an approach is suggested by U.S. Pat. No. 5,606,286 that issued to Burns. This approach requires additional complexity in electronic circuitry. The third approach is to utilize only frequencies, which reduce the occurrences of IM products by a frequency planning procedure. U.S. Pat. No. 5,295,138, issued to Greenberg, et al., and assigned to Northwest Starcon Limited Partnership uses this approach. The ""138 patent reduces the effects of IM products within a common communication channel bandwidth by determining carrier frequencies, which are contained in this bandwidth, so that the IM products are reduced. The ""138 patent adjusts carrier frequencies, performs measurements of the resulting IM products, and readjusts the carrier frequencies based upon the measurement results. The ""138 patent addresses a problem encountered in satellite radio systems in which the assigned frequencies are randomly selected and moved in an iterative process to reduce the effects of IM products. This approach, however, does not address situations in which frequencies are reused as with cellular radio systems.
Moreover, current literature in the field of cellular radio leads away from the present invention. For example, one reference states that the effects of IM products xe2x80x9cwill not affect channels within the transmitted band designxe2x80x9d when considering the RF combiner with respect to the relatively low amplitudes of the IM products that are generated. (Lee, William C. Y., Mobile Cellular Telecommunications Systems, McGraw-Hill Book Company, 1989, pp. 231-232.)
Even though interference in a cellular radio system is a well-recognized problem (any increase of interference will degrade the call capacity of the cellular radio system), the sources of this interference may not be well defined. The current art of cellular radio recognizes co-channel and adjacent channel interference as being two sources of interference. Both co-channel and adjacent interference are addressed by the frequency planning practice detailed in a number of references, e.g. Boucher, Neil, J., The Cellular Radio Handbook, Quantum Publishing, 1995, pp. 116-118. The current state of the art propounds that regularly spaced channel sets, each channel set containing a plurality of channel numbers, be assigned in a repeated fashion to cell sites within the coverage area of the cellular radio system. This approach to frequency planning for a cellular radio system fails to recognize the effects of IM products as a source of interference. Reducing the interference attributable to IM products reduces the total interference experienced at the mobile subscriber unit and at the base station.
The problem of reducing the frequency reuse factor by reducing the effects of intermodulation (IM) products in a cellular radio system is solved and a technical advance is achieved. The invention consists of determining a channel set for each antenna sector of each base station and of assigning an appropriate channel number of the channel set to the associated base station equipment and mobile subscriber unit in order to support a cellular call. Channels are chosen so that the measure of homogeneity is reduced, thus reducing interference associated with IM products. Consequently, the total interference is reduced, allowing for a smaller reuse frequency factor and enhancing the call capacity of the cellular radio system. If the call requires a handoff from the serving base station to another base station or from one antenna sector to another antenna sector of the serving base station, the cellular radio system assigns another channel number, as determined by the invention, so that the call can continue by being served by the second base station or by the second antenna sector.